A novel vanadium–titanium redox flow battery with a mixed electrolyte approach

Abstract

Redox flow batteries (RFBs) enable independent scaling of energy and power, making them a suitable candidate for grid-scale energy storage solutions. However, the market is currently dominated by vanadium RFBs, which are prone to extreme price volatility. To reduce the inherent material costs while retaining the efficiencies, the study investigates a novel vanadium–titanium RFB using (i) a separate, acid-based system (V₂O₅ + H₂SO₄ catholyte, and TiCl₃ + HCl anolyte) (system-1), (ii) a premixed acid-based electrolyte at 0% state of charge (system-2), and (iii) an ionic-liquid-based premixed electrolyte comprising [Bmim]Cl + VCl₃ and its titanium counterpart (system-3). Several physical and electrochemical characterization studies were performed, such as viscosity and density measurements from 298 to 333 K, cyclic voltammetry tests, and electrochemical impedance spectroscopy to establish a stable redox window for V(IV/V) and Ti(III/IV) in the presence of an ionic-liquid. In galvanostatic cycling with a SPEEK membrane, system-1 delivers coulombic efficiency (CE) greater than 98%, at 10–20 mA cm⁻², with an energy density of 24.53 Wh L⁻¹. System-2 (premixed acid) retains the CE%, while the voltage efficiency (VE) drops by 10%. The results demonstrated that premixing at 0% SOC effectively limits the crossover in V–Ti RFBs, while optimization of proton donors and the inclusion of additives are necessary to enhance the nominal discharge potential.